Of oxide high temperature superconductors, a Cu family superconductor thin film (see Kotai Butsuri (Solid State Physics), Vol. 35, No. 5, 2000) excels in superconducting properties, and research and development efforts have diversely been devoted to putting it to practical use. The excellent superconducting characteristics which a Cu family superconductor thin film possesses include the feature that it excels in high-frequency properties as mentioned in the literature referred to above. In order to fabricate a superconductor thin film that can be applied to a high-frequency device such as a microwave device, of importance is not only the high-frequency properties of the superconductor thin film itself but also those of a substrate on which the superconductor thin film is to be epitaxially grown.
Fabricating a Cu family oxide high temperature superconductor that excels in superconducting properties requires the superconductor thin film to be excellent in both crystallographic integrity and crystallographic orientation.
For a conventional Cu family superconductor thin film, use has been made of a SrTiO3 substrate that is small in lattice mismatch with such a superconductor thin film and as a result is capable of growing thereon such a superconductor thin film that is high in crystallographic integrity and excellent in crystallographic orientation. However, since it is large in dielectric constant (specific dielectric constant=about 300), SrTiO3 is unsuitable to form a substrate for growing a superconductor thin film thereon for particular use in a high frequency device.
Thus, in order to be applicable to a high frequency device, there has been sought an oxide high temperature superconductor in which an oxide high temperature superconductor thin film that is high in crystallographic integrity and excellent in crystallographic orientation is formed on a substrate that is low in dielectric constant, and a method whereby such an oxide high temperature superconductor thin film can be simply epitaxially grown on such a substrate.